Heat treatment of steel



Patented 14, 1944 PATENT OFFICE HEAT TREATMENT OF STEEL Welton J. Crook,United States Army,

Davenport, Iowa Application April 8, 1942 Serial No. 438,102 1 Claim.(Cl. 148-215) (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) v The invention described hereinmay be manuiactured and used by or for the Government for overnmentalpurposes, without the payment to me of any royalty thereon.

The present invention relates to the heat 5 treatment of steel, bothplain and alloy, and more particularly to a new and improved method ofenhancing the physical properties of abnormal steel by means of aspecial heat treating process. i

As is well known in the art, the operation of normalizing consists inheating steel to some temperature above the upper critical temperatureand below 2000 F., for a time sufiicient to produce a state of solidsolution of all contained carbides, followed by cooling in still air.This procedure is commonly applied to heavy forgings, steel castings andsimilar products, prin- .cipally for the purpose of homogenizing theerally as a preliminary treatment which is followed by heating,quenching and drawing. Nor-. malizing followed by reheating below thelower critical temperature is also well known as a method of removingcoolingstresses produced by normalization.

.Heretofore, where the properties required in steel were above thosenormally obtained in the as rolled or as cast" state, the steel hasgenerally been subjected to heat treatment by heating the same to sometemperature above the upper critical limit, quenching in oil orwater andthen reheating to some'temperature below the critical point, thereheating or draw treatment being adjusted with reference to theparticular properties desired. This quench and draw procedure, while ofdefinite value in improving the properties of the steel, is neverthelesscharacterized by certain disadvantages. Thus, for example, the quenchingoperation produces somewhat variable results depending on the mass ofthe objectbeing treated, particularly where the object has sections ofwidely difierent sizes, in which cases uniformity of'results is yieldstrength and superior machinabllity.

exceedingly difficult if not impossible'to attain. Moreover, themachinabillty of the ultimate product is not all that could be desired.Again, the usualtreatment of quench and draw often results in thedevelopment of residual stresses which appear after partial or finishmachining, especially in objects which require a high degree ofstraightness or dimensional stability. Warpage is also undesirably highas a result or the quench and draw operation, and, lastly, considerableequipment is required by the prior art processes.

One of the objects of the present invention is to provide a new andimproved process of enhancing the physical properties or steel, whichobviates the foregoing disadvantages of the prior art quench and drawprocess.

Another object is a process of the character described wherein theuniformity of the results obtained is not adversely aflfected either bythe configuration or by the mass of the article being treated.

A further object is a heat treating process which permits the attainmentof a given set of enhanced physical properties and especially Brinellhardness or its' equivalent, together with a greatly superiormachinability in the ultimate product, compared to the machinability ofa product of comparable physical properties produced by the prior artquench and draw procedure.

A still further object is a process of the character hereinbeforeindicated, which eliminates to a high degree the presence of residualstresses and warpage tendency typical of steel objects which have beentreated by the usual queue and draw process.

Another object is to provide a steel of improved physical properties,characterized by high Other objects will be apparent as the invention ishereinafter more fully described.

- The foregoing objects may be accomplished in accordance with thepresent invention, one

aspect of which comprises a special heat treating process applicable toany steel, whether plain carbon 01' alloy, which exhibits a condition ofsubstantial abnormality under the condition of the McQuaid-Ehn test. Inaccordance with the present invention. such abnormal steel is firstnormalized and thereafter drawn at a temperature below the lowercritical temperature thereof and within the range of approximately500'F. to approximately-1200 F. The thus treated steel is characterized byan enhanced yield strength and superior machinability. as hereinaftermore particularly pointed out.

The present invention is based onthe discovery that, when operatingunder certain conditions and upon a class of steel which mayconveniently be designated as reactive," a draw treatment following anormalizing operation produces an altogether unanticipated andthusi'arinexplicable enhancement of the physical properties of the treatedsteel, rather than the expected impairment thereof. Indeed, when a steelof a reactive type is normalized and then sub- .iected to a reheatingtreatment below the lower critical temperature. for example,-at 500 F.,600' I". and up to about 1000" F., it has been found that the higher thereheating temperature, the

ably, but not necessarily, should have a McQuaid- Ehn grain size rating(A. 8. T. M.) of-from about six to about eight. Such steels whennormalized assume a characteristic fine-grained acicular ferritestructure wherein the carbide particles-are rejected at the grainboundaries. "Abnormal" steel possessing the foregoing characteristicsexhibits the peculiar property. not by "normal" steel, of respondingfavorably to 9. normalize and draw treatment as hereinafter described:i. e., the yield strength of the abnor-' mal" steel is thereby enhanced,whereas the physical properties of "normal" steel, on the other hand,are either unaflected'or impaired.

In carrying out the process of the present invention, a "reactive" steelis first normalized in any convenient manner well known to the art, i.e., it is heated to some temperature, above the upper critical limit andbelow a temperature vof about 2000 F., fora time suflicient to produce astate of solid solution of all contained carbides.

and then cooled. freely in still air. The second part of the treatmentcomprises a drawing operation, wherein the normalized "reactive" steelis reheated to a temperature below the lower critical temperature andwithin the range of approximately 500 F..-to approximately 1200' F..preferably from about 600 F. to about 1000' I".

It will thus be apparent that the process requires no special apparatus,and may be simply and readily carried out with less equipment than that1.

required for the usual quench anddraw process. In .order more clearly todisclose the nature of the present invention several specific examples,

will hereinafter be described. It should be understood, however. thatthis isdone solely by way of example;'and isnot to be construed as aTssu: II

Elan! I u Tensile Yield Red. llrinell str. 'str. m? ares hardnessamo'rrva BteeLA- d P.|.|. P.s.i. Percent sreoe VG }11a,s00 07,000 10.1no 22.1 "rat s I .norm. }i04,s00 15,200 22.2 02.5 207 NoN-nmc'rrvs.

' Steel B-fll oeivcd 100.s00 00.000 20.0 40.3 1111 .110 sm l g joam00,000 21.1 41.4 001 r48 ccived 114,000 00,300 10.0 01.0 211 was? Q.110, uwmrm }101.10 0 05,000 22.0 ass 211 limitation upon the spirit orscope of the appended claim.

when a "reactive" steel is normalized and thereafter subjected to'areheating treatment below the critical temperature and within the rangeof from about 500 1". up to approximately 1000' F., it has been foundthat the hlsher the draw temperature, the greater the enhancement in theyield point. This effect of the draw temperature may be seen byreference to Table I. t

has: I Eflect o/ normalizing and-draw temperature 011 abnormal WD2840steel 1 Norm. Draw 'len. Yield E a Red. llrincll temp. temp. ltr. str.fg; area hardiness r. r. P. 0.1. P. '1. 4. Percent 1. 050 1, 100 102, SM67, s00 fl-O 57. 3 107 1.5) 000 111.1! 70.250 19.6 51.3. 223 1,8) 700111,250 11.500 21.3 50-5 229 1.000 000 111,000 70,000 21.0 01.3 212 1,800 1, 103. 70. M M. 2 B6. 2 212 If the draw temperature materiallyexceeds about 1000" F. (for example, above about 1200 F.) there is. inmany cases, a diminution in yield strength of the normalized and drawn"reactive steel. However, there is also a corresponding increase inelongation and reduction of area, and

therefore, in practice, the draw temperature may in some instances bedesirably carried as high as 1100' F., for example, in order to attain aselected combination of physical properties.

In the case of a non-reactive" steel (e. 8., one which issubstantially"normal" under the conditions of the McQuaid-Ehn test) the samenormaliz'e and draw procedure which produces enhanced physicalproperties in a "reactive" steel. results in many instances in animpairment of yield strength in normalized and drawn "nonreactive steel.The striking difference in the response of "reactive and,"non'-reactivesteel to a normalize and draw treatment is illustrated by the data givenin Table 11.

Referring now to the six figures or the accompanying drawings.

Figs. 1 to 3- are reproductions of photomicrographs of a typical normal,non-reactive" aasaaaa .WD2340 steel, as rolled (Fig. 1), normalized(Fig. 2) and normalized and drawn (Fig. 3).

Figs. 4 to 6 are similar viewsof a typical abnormal reactive" WD2340steel, as rolled (Fig.4), normalized (Fig. 5) and normalized and drawn(Fig. 6).

Figs. 1 to 3 shows clearly that no useful reaction has taken place inthe case of the normal steel, whereas Figs. 4 to 6 illustrate thechanges effected in an abnormal steel by thesteps. of the hereindescribed process. of the two steels shown in Figs. 1 to 6 were veryclosely similar as far as carbon, manganese, phosphorus, sulfur, siliconand nickel are concerned. The "reactive steel, however, showedspectographic. traces of aluminum, whereas the "nonreactive steel didnot.

The manner in which the condition of abnormality" in a reactive steel isproduced or brought about forms no part of the present invention,inasmuch as any suitable procedure, well known in the prior art, may beused for this purpose. Thus, for example, the condition may be createdby suitable aluminum additions, to the steel. Other grain-controladjuncts may also be used, either singly or in combination, if desired.Where aluminum is employed, however, there are several reasons whyneither a definite aluminum addition can be specificed nor a definitepercentage of residual aluminum oxide can be required in a finishedsteel in order to assure that condition of substantial abnormality whichprovides the convenient index of "reactivity herein employed.

The chemical analysis In the first place, the effect of definitealuminum 1 additions to steel for purposeof grain size control dependslargely on the particular melting practice employed and is greatlyinfiuenced'by the state of deoxidation of the metal before the aluminumis added. In other words, difierent melting practices may require moreor less aluminum to produce a given degree of grain size control.Moreover, the amount of aluminum oxide remaining in grain sizecontrolled steel is generally exceedingly small and an accurate chemicalanalysis with respect thereto is difficult to obtain. For these andother reasons, the specification of a given grain size range is a morereliable and indeed more fundamental means of assuring reactivity insteel than any attempt to specify either the amount or character of thegrain size control adjunct. available tb'the steelmaker for the purposeof attaining the required condition of substantial abnormality. Inshort, in the procurement' of a reactive steel, it is preferable toleave it to the steel maker to attain a specified grain size accordingto his particular furnace practice, and not to limit his choice of grainsize ,control procedures available to him.

In the case of certain steels containing moly-bdenum, it has beenobserved that the herein described normalize and draw procedure willeffect a slight improvement in physical properties even when the steelis only partly abnormal, However, substantial improvement in yieldstrength is generally attained only in steels exhibiting a condition ofsubstantial abnormality under the McQuaid-Ehn test. For these reasons,

rating of, say, from six to eight (A. S. T. M.).

EXAMPLES specific examples will now be described in detail.

the preferred reactive steel has a grain size The chemical analysis,maker's name, "WD number" (War Department specification number) physicalproperties developed by each steel when normalized at 1800 F. and drawnat 1100 F. are shown in Tables III and IV.

TABLE III Chemical composition of McQuaid-Ehn test 1 samples BpecimenNo.0 Mn P 8 Ni Cr Mo Per- Per-. Per- Per- Per- Per- Percent cent cent centcent xe'nt cm! 0.40 0.69 0.016 0.015 3.44 0.42 0.74 0.016 0.014 3.52

040 0.95 "Trace 0.38 0.86 ..Trace TABLE IV Description 0! samplessubaectcd to McQucud-Ehn test Norm. and drawn Phys- Pro I i Maker W icalyield 1 No. No. ten Q 1*.long.|

- pcr- R..\. 111m cent2t l ilercenl an! 55.9 207 20.4: 51.9 207 19.355.7 212 111.21 55.1 207 111.51 37.8 217 am, 54.1 201 21.2: 135.9 20720.51 49.1 217 22.2! 55.1 207 10.0: 45.4 223 22.3 511.2 -22s 23.71 55.0207 25.3 58.5 223 22.9 551 196 22.7 47.4 22.8 56.7 202 15.7 46.2 26918.9 50.2 228 .17.6 45.4 255 10.6] 58.6 235 Republio. 'limken. Carnegla.Copperweld.

In performingthe McQuaid-Ehn test, samples of each of these steels werebox carburized for eight hours at 1700 F. and cooled in the box in thefurnace. The rate of coolingfrom the carhurizing temperature wasregulated so that at least three hours were taken in coolingthrough Tm:V

Result: from HcQuaid-Ehn tests wD N m Reaction Graitiii sire orm y tonorra ng number rating malize and A. S. T. M.

' draw No.

4140 A Yes 5 4140 A Yes 5 2340 A No 1 4150 A No 3 2340 A Yes 2-6 2340 BYes 8 2340 B Yes 1 6-7 4140 B Yes 1 5-7 2340 B Yes 4 2340 B Yes 7 2340 BYes 6 2340 D Yes 7 2340 B Yes 6 2340 B Yes 8 2340 B Yes 6 2340 B Yes I7-3 2340 B Yes 7 3240 i 13 Yes 8 Q40 Yes 8 4150 0 Yes 7 I Duplex.

TABLI VI Limiting properties to be developed in normalized'and drawnsteels Elong. Ten. Yield Rod. Sm! str. str. area P. I. i. P. r. l.Percent WD 2340 100,000 70, (D0 18 30 WD 4140 1 I05, 000 80, 000 18 30 1Including WDl040, 3140 and 3240 steels. Including WD4150 steel.

From an inspection of Table V it will be evident that a reactive" steelis preferably one having a McQuaid-Ehn grain size rating of from six toeight. g

In general. steels which do not contain molybdenum must bepartlyabnormal or "abnormal" in order to assurethe so called "molybdenumstructure when normalized, and this structure has been found to becharacteristic of all steels which are "reactive to the normalize anddraw treatment. In the case of-steel which contain molybdeumn (forexample WD4140 steels), slight "reactivity" is noted in some cases, evenwhere the steel is normal or only partlyabnormal. It is probable,however,gthat a degree of abnormality must exist in all steels if theyare to respond favorably to the normalize and draw treatment. In otherwords, a condition of substantial abnormality is correlated withsubstantial reactivity" within the meaning of the present invention,lesser degrees of abnormality being correlated with lower degrees ofreactivity.

Numerous advantages are realizedby the prac' tics of the presentinvention. Thus, for example, the process of .the present invention iseasier to apply than the usual process of quench and draw, and requiresless equipment. Moreover, when various sections or sizes of material areto be treated. the effect of mass on the uniformity of the resultsobtained does not intervene as it does in the quench and draw treatmentprior art.

A further important advantage of the herein described process ascompared to the quench and draw process, relates to the machina-bilityof the I ultimate product. It has been found in numerone tests that, fora given set of physical properties and especially Brinell hardness orits equivalent, the machinability obtained by the present process isgreatly superior to that obtained by a quench and draw treatment. Thisimprovement may be illustrated by a test performed on two sets of fiveinch steel bars selected from the same heat of "abnormal" WD2340 steel,one set being heat treated by oil quenching and drawing, the other setbeing treated in accordance with the present invention, both having thesame hardness, namely, 286. Brinell. Both sets of bars were then drilledin a ver-'- tical Stecher drill. on the average, it required three (3)drill blades to drill each of the oil quenched and drawn bars, whereasone blade was capably drilling three (3) of the normalized and drawnbars.

Another advantage of the present invention was noted in the machining ofobjects requiring a high degree of straightness or dimensional rigidity,as for example, in the case of rods and tubes for gun recoil mechanism.It was found that the normalize and draw treatment elimimated to a highdegree the development of residual stresses which often results afterpartial or finish machining of similar objects which have been quenchedand drawn. It was also notedthat warpage in steel which has been treatedby normalize and draw is greatly de-' creased and practicallyeliminated.

In the foregoing detailed description of the present invention it willbe apparent to those skilled in the art that many variations may be madewithout departing from the spirit and scope thereof. I therefore intendto be restricted 'only in accordance with the following patent claim.

I claim:

The two-step process of producing steel gun recoil cylinders,counter-recoil cylinders, hollow control rods, and the like, whichcomprises the steps of normalizing said steel, and thereafter drawingsaid normalized steel below the lower critical temperature thereof andwithin the range of approximately 500 F. to approximately 1200 F., saidsteel being selected from the group exhibiting a condition ofsubstantial abnormality under the conditions of the McQuaid-Ehn test andhaving a grain size rating (A. S. T.-M.) of from six to eight,inclusive, whereby the physical properties of said steel are enhancedwithout impairing the machinability thereof by hard spots due to theformation of martensitlc structure and whereby stress relievingoperations are substantially eliminated due to the absence ofdistortion.

WELTON J. CROOK.

of the o Certificate of Correction Patent No. 2,343,888. March 14, 1944.

' g I WELTON J. CROOK i It is hereby certified that error appears in theprinted specification of the above numbered patent requiring correctionas follows: Page 3, second column, lines 35 I a to 40 inclusive, theheading to Table IV should appear as shown below instead of as in thepatent Physical prop. norm. and drawn BpeclNo. Maker W!) No. Ten. str.psi. Yield m. psi. fi R. A. percent BHN and that the said Letters Patentshould be read with this correction therein that the same may conform to"the record of the case in the Patent Ofiice.

Signed and sealed this 6th day of June, A. D. 1944.

[sun] LESLfi] FRAZER,

. Acting Commissiom of Patents.

